Satellites and other spacecraft are in widespread use for various purposes including scientific research and communications. These scientific and communications missions, however, cannot be accurately fulfilled without uploading and downloading electronic data. In many applications, the satellite relies upon uploading and downloading electronic data for position correction or system diagnostics. In other applications, the satellite acts as a relay to transfer data between two remote points on the Earth. Without the capability to receive and transmit electronic data, proper satellite function is hindered and at times impossible.
Many modern spacecraft act as relays to transfer data between two or more points on the Earth. There are several types of electronic data that these satellites broadcast. These include timing beacon synchronization data, multi-cast/broadcast data service, calibration data, and point-to-point data service. Unfortunately, typical satellites require multiple antennas and the associated electronics (modulators with frequency division multiplexing plans) required for each antenna for each of the above functions. The partitioning of satellite resources (capacity, power) across the services is either fixed of selectable in fixed quanta, and the services themselves often realized through parallel chains of RF and antenna equipment on the satellite. Furthermore, the terminals in these systems require multiple parallel receivers to participate in different transmission services.
For example, a satellite beacon signal is required by a terminal to synchronize time for a TDMA satellite communications system. The beacon signal typically has its own waveform and is generated independent of data transmissions. To conserve satellite power, the beacon is allocated relatively low EIRP. To conserve spectrum, the beacon occupies a small fraction of the downlink bandwidth. To minimize interference with the main data transmission, the beacon is usually located at the edge of the allocated band.
In addition to the added complexity in the generation and recovery of the beacon signal, the conventional approach has a number of inherent problems. First, there is a concern with possible interference from the main data transmission. Second, a small bandwidth can limit the timing accuracy of the beacon. Finally, the timing biases between the data transmission and the beacon signal due to different hardware paths typically need to be calibrated and eliminated.
The disadvantages associated with these conventional satellite transmission techniques have made it apparent that a new technique for satellite multimode transmission using time division multiple access is needed. The new technique should reduce the number of antennas and modulators required in the satellite while improving transmission performance. Additionally, the new technique should reduce the number of antennas and demodulators required in the terminal and allow a single terminal, with a single antenna, to receive all types of satellite service. The present invention is directed to these ends.